Is "La Belle Dame Sans Merci" a successful ballad?
I think we need to revise some poetic terms here. I personally wouldn't describe this excellent poem as a ballad. Let us remember that a ballad is a story that is told in song. The majority of ballads are characterised by a strong meter and a refrain. Ballads normally feature the kind of stories about love, vengeance and violence that have been passed down through the generations and also use rather formulaic descriptions which made them easy to remember by the singers.
Now, if we have a look at this poem for one moment, we can see that although the meter is on the whole regular, it does not have the necessary regularity that would make it appropriate to be sung. In addition, there are no repeated refrains as in other ballads. Although the first and last verse to a certain extent mirror each other, ballads normally would have one line or two lines that are repeated at many different stages throughout the poem. Lastly, we cannot say that formulaic or cliched phrases are used in the poem to describe the lady or the knight in the same way as in ballads.
Therefore it appears that we cannot really describe this poem as being a ballad and therefore it is not a "successful ballad" as your question asks. It is more accurate therefore to describe it as being a narrative poem that tells a tale. A narrative poem is similar in many ways to a story, as it has one or more characters, provides us with a concrete setting, gives us a conflict and a series of events.
A "ballad" is defined as a "relatively short narrative poem, written to be sung" or recited. Is "La Belle Dame sans Merci" a ballad? Literary critics note that Keats imitates the form of a folk ballad in this poem with his use of simplistic language that describes a unique event, but provides only a limited number of details. The author also does not lead the reader to form a particular opinion, but simply conveys the "story."
The rhythm and rhyme are ballad-like, with lines that alternate in length and the use of a specific rhyme scheme. With this said, I would suggest that because of Keats' intent is to create a ballad-like poem, it would be more more the ballad rather than less because it is contrived. As the poem employs many of the characteristics of a folk ballad, I would suggest that Keats is successful.
Formulaically, "La Belle Dame" has the form of a ballad with some variation to the form. As per the form for ballads, it tells of death, the supernatural, and death. It has ballad stanzas that are quatrains (four lines). It has alternating repeating lines that are in iambic tetrameter (4: ^/ ^/ ^/ ^/) followed by iambic trimeter (3: ^/ ^/ ^/). One line in the last stanza that is in pentameter, with a headless first foot, and reads: "Though the sedge is wither’d from the lake, ...." Some stanzas, like X, vary the form by having ending lines of dimeter (2: ^/ ^/): "Hath thee in thrall!". A literary ballad that is an intentional imitation of the anonymous folk ballads, "La Belle Dame" follows in a popular poetic tradition of writing literary ballads practiced by the Romantic poets and succeeds formulaically and emotionally and poetically.
Literary ballads grew out of an increasing interest by social elites and the Romantic poets. As demonstrated in post #3, "La Belle Dame sans Merci" is, indeed, a successful ballad. Moreover, it succeeds thematically as well as a melancholy lament over the impermanence of a life experience; in this case, beauty.
Ballads are lyric poems that also tell a story. What makes them ballads is the musical quality. They can be sung. I guess you can argue that if you can sing it, it can be a successful ballad. I am not sure anyone can sing this, but I have seen people sing all kinds of poems so I suppose it's possible.
I must agree with the previous posts. The poem in question is not typically considered a ballad. That being said, regardless of its categorization, the poem is successful, for me, because it conveys movement, emotion, and possesses poetic devices typical of the period.